Bicycles are used as convenient means of transit. However, the trouble is that commuters or day students may be visited by sudden puncture. Another trouble is that they need to frequently fill air into inner tubes. Accordingly, a request of being released from maintenance of tire wheels grows bigger and bigger. Further, there is a growing tendency to use bicycles at a state of emergency such as an earthquake disaster.
Under these circumstances, tire wheels commonly called puncture-free tires have come to be investigated which, for example, comprise a solid tire in place of a tube-containing tire or a tire having a rubber foam with which the tube-containing part is filled. Structures of such tire wheels have been proposed since long ago. For example, JP-B-U-40-11446, JP-A-47-26476, and JP-A-57-155101 disclose a technique of producing a puncture-free tire wheel by filling the space of a tire body with an elastomer, a soft rubber layer, etc. (The term "JP-B-U" and "JP-A" as used herein mean an "examined Japanese utility model publication" and an "unexamined published Japanese patent application", respectively.)
However, the conventional tire wheels called puncture-free tires have a drawback that the filling material has a large specific gravity (usually not smaller than 0.4) and this makes the tire wheels heavy and difficult to handle, although puncture mending can be avoided.
Another drawback of those conventional tire wheels is that because the hardness of the solid rubber or rubber foam is usually as high as 40 degrees or higher, the bicycles employing the conventional wheels have poor shock absorption during riding and hence are uncomfortable to ride on. The hardness of a solid rubber herein means the type A hardness prescribed in JIS K-6301, while the hardness of a rubber foam herein means the ASKER type C hardness prescribed in Japan Rubber Association Standards SRIS-0101.
A further drawback is that since the conventional tires have a low impact resilience of lower than 50 (as determined by JIS K-6301), they have high rolling resistance and this makes bicycle riding laborious.
Because of the drawbacks described above, those proposed ideas have failed to be put to wide practical use.
Although a puncture-free tire containing a polyurethane or ethylene-propylene-diene rubber (EPDM) sponge fitted into a tire body has, of course, been put to practical use, this sponge-filled tire is still insufficient in the mitigation of the above-described drawbacks and is inferior to air-filled tires because of these problems. Even if the sponge fitted into the inner space of tire body is a closed cell type one, when the sponge is made from a material such as EPDM, air leakage from the closed cells is apt to occur.